The invention relates to a method and device for stabilizing a paper web in a group of cylinders in a dryer section of a paper machine. The group of cylinders comprises a row of heated drying cylinders and a corresponding row of leading rolls or equivalent placed in interposed interlocking relationship with the drying cylinders. The heated drying cylinders in the group are placed outside a loop of a drying wire whereas the leading rolls in the group are placed inside the loop of the drying wire such that a single-wire draw is defined between the leading rolls and the drying cylinders. A stabilization device is provided in the area between adjacent drying cylinders and one of the leading rolls or equivalent which is placed in proximity to, and in an interlocking relationship with, the adjacent drying cylinders in the group of cylinders. The interlocking relationship provides that the drying wire runs from a first one of the adjacent drying cylinders to the leading roll and then to the second one of the adjacent drying cylinders. An air flow is directed from the stabilization device in a direction opposite to the running direction of the paper web.
Prior art dryer sections in paper machines comprise a number of drying cylinders, which are usually heated by steam. The paper web or board to be dried is pressed against the drying cylinders by means of the drying wire into contact with a heated face of the drying cylinders. In the dryer sections of prior art devices, both a twin-wire draw and a single-wire draw of the drying wire are employed. Recently, the single-wire draw has become more common because it allows the web to be dried with constant support such that there is a closed draw throughout the entire dryer section and the web does not have any free draws.
Generally, dryer sections with a single-wire draw and two rows of cylinders are used. In these dryer sections, the cylinders in an upper row are heated cylinders and are placed outside the loop of the drying wire. The cylinders in a lower row are leading cylinders or rolls which are provided with suction holes for promoting the support contact between the web and the wire. This result is desirable because the web is running on an outer surface of the drying wire over the leading rolls.
When a number of the above cylinder groups with a single-wire draw are employed and arranged one after the other, it is a significant drawback that the web is dried unevenly, i.e. more quickly at the side that is placed in direct contact with the heated cylinder face. This causes an asymmetric drying of the web.
Asymmetric drying of the web produces a number of drawbacks. For this reason, in recent years multi-cylinder dryer sections having a single-wire draw have become more common in which so-called inverted cylinder groups are used. In an inverted group, the drying cylinders are placed in the lower row and the leading cylinders or rolls in the upper row.
However, in these inverted groups, a significant drawback results from the pumping action caused by the wires because the pumping of air produces detrimental pressure on the free portion at the inlet side of the wire and in the closing wire nips. In the wedge spaces that form the inlet-side nip for the web and for the wire, a pressure tends to be induced. Corresponding opening nips produce detrimental negative pressure because replacement air is sucked into the nips from the sides of the dryer section. The in-flowing air attempts to penetrate between the wire and the paper thus separating the edge of the paper from the wire. This effect causes several drawbacks, such as web breaks.
Moreover, in an inverted group, the negative pressure in the outlet nips is increased further by the so-called chimney effect, i.e. by the air flows that can rise unhindered out of the spaces upwards by the effect of gravity.
It is also known in prior art devices, in particular in multi-cylinder dryers having a single-wire draw, to place the drying cylinders and the leading rolls very close to one another so that a more compact and less expansive paper-machine dryer section with a single-wire draw is obtained. In this regard, the dryer section can be made both shorter in the machine direction and lower in the vertical direction as compared with other prior art devices. By means of this arrangement, economies are also obtained in the cost of the paper-machine hall. However, in such groups of compact geometry in dryers of paper machines, problems are caused by winding of the paper onto cylinders and rolls. Consequently, the distance between the drying cylinder and the leading roll must be sufficient for the passage of loose paper and in particular for the passage of paper clods. Such a safety distance is commonly about 50 mm to about 100 mm.
In addition, in prior art device such as those described above, the distance between the blow-boxes, stabilization tubes, and equivalent used in the dryer section and the drying cylinder/leading roll must, for the same reasons explained above, be sufficient to pass loose paper and paper clods thereby avoiding unnecessary interruptions. The blow-boxes, stabilization tubes, and equivalent, are supposed to prevent effects detrimental to the support contact between the web and the drying wire.
The phenomena that interferes with the support contact between the web and the drying wire arises, for example, from the fact that boundary-layer flows produce pressure differences between the different sides of the drying wire. These problems and the solutions related to them have been discussed, for example, in Finnish Patent Nos. 65,460 and 69,332.